2.5" sensor usually means something like 1/2.5" sensor, that is rather small... sensor makers have a funny way of relating inches to sensor size which I have never truly figured out.

For example, Nikon 1 has a "1 inch sensor", and the actual size is 13.2x8.8mm, I wonder where 25.4mm fits in there?

When 2.5" sensor is talked about in compact cameras the size is 5.76x4.29mm, but I doubt it would be that tiny. But I also find it highly unlikely that it would be 63.5 in diagonal. Is there an official statement of the actual exact measurements of the sensor?

Iīm pretty sure this is what it says, the camera is much too big for a small 1/2,5" and it sports full PL mount which also doesnīt make sense for such small stuff.The datasheet is definite - it says 2,5".

Iīm pretty sure this is what it says, the camera is much too big for a small 1/2,5" and it sports full PL mount which also doesnīt make sense for such small stuff.The datasheet is definite - it says 2,5".

On the other hand, why would the camera have a CP.2 lens on it? Does it actually cover the supposed 63x35mm format, twice as wide and 50% taller than the intended image circle?

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2.5" sensor usually means something like 1/2.5" sensor, that is rather small... sensor makers have a funny way of relating inches to sensor size which I have never truly figured out.

For example, Nikon 1 has a "1 inch sensor", and the actual size is 13.2x8.8mm, I wonder where 25.4mm fits in there?

Ahh! Bingo! It is related to the TV tube inch measurement, not in fractional inches, but indeed in whole inches.

1" is technically defined as 12.8 x 9.6mm, which means that multiplying the size by 2.5x results in a size of 32x24mm, making it smaller than 35mm format in the horizontal dimension, and the final image is likely going to be smaller still, because it will use a recording ratio typical for video rather than 4:3.

All in all, the sensor size physical recorded image will be somewhat bigger than the standard S35 of cinema cameras, and even the Epic sensor which is 30mm wide. This means that not all PL lenses will work, and why the CP.2 is demoed, full-frame lenses are likely to have better coverage for the whole sensor.

My guess is that this "2.5inch" is in the usual bizarre convention of video sensors, where the actual diagonal is about 2/3 of the stated number, like 4/3" meaning 22.5mm diagonal. So scaling up: 22.5mm * (2.5" / 4/3") = 42.2mm, fairly close to the 43.2mm diagonal of 36x24mm "35mm format". That seems a good size choice for a camera of such high resolution, which is likely to want access to higher resolution still-camera lenses.

In case anyone is wondering, for curiosity's sake, the inch size was developed way back before the 90's when video cameras used CRT tubes to capture the image instead of CCDs. Tubes were typically measured in terms of diameter in fractional inches, but the useable area was only a central portion of the tube, so for any given size, a smaller center crop was used.

Since by the late 80's a whole industry was already built around the size convention of tubes, it made sense to keep it when talking about digital sensors, so a 2/3" lens could stay 2/3" and not some new made up figure.

I do wonder why NHK used such an outdated sizing method for a large sensor camera.

This sensor is described at 2.5", not 1/2.5" --- a size not on that list. By the way, some Sony data sheets for its "APS-C" sized sensors describe them with wording like "APS Size Diagonal 28.4 mm (Type 1.8)",where "Type 1.8" s a synonym for the weird '1.8 inch' specification:http://www.sony.net/Products/SC-HP/cx_news/vol28/pdf/icx413np.pdf

The size in fake inches or "Type" only refers to the diagonal measurement. For example, there are 2/3" sensors in different shapes, some 4:3 for stills, others 16:9 for HD video, with the common dimension being the diagonal of about 11mm.

2.5" is 63.5mm, or 42.3mm diagonal according to tube sizing, which would result either in a roughly 36x20mm 16:9 sensor or 33x24.75mm 4:3 sensor, the latter of which sounds more practical from a manufacture point of view.

4:3 is not impractical for video, because as we can see in the Alexa, the extra sensor space can be used to emulate the full area of 35mm film stock, which allows the use of anamorphic lenses without any crop. With Cooke having recently announced a line of anamorphic lenses, we may just end up seeing more video cameras that have 4:3 sensors.